config: Use --program-prefix=eu- by default.

config: Use --program-prefix=eu- by default.

We want eu- as default program prefix if none was given by the user.
But if the user explicitly provided --program-prefix="" then pretend
it wasn't set at all (NONE).

This makes sure all tools will be installed as eu-<program>.
We want this default since most tools would normally conflict with
the GNU binutils variants that have similar names.

Signed-off-by: Mark Wielaard <mjw@redhat.com>